This invention relates to porous separators which are disposed between the electrode plates of a battery.
Batteries include a plurality of electrode plates which are arranged to provide alternating positive and negative electrodes. Separators are included in the batteries to prevent direct contact between plates of opposite polarity while freely permitting electrolytic conduction.
Battery separators are made from an insulating porous material and in general must possess certain properties. The separator medium must be resistant to degradation and instability in the environment of the battery, such as degradation e.g. by strong acid solution at ambient and elevated temperatures and strong oxidative attacks. The separator would also be capable of allowing a high degree of ionic movement, i.e. it should have a low electrical resistance. The separator would also be capable of inhibiting the formation of conductive paths between plates and consequent shorting. This latter problem can arise during battery operation when parts of the battery electrode become dispersed in the electrolyte and precipitate or become deposited in the separator.
U.S. Pat. No. 3,351,495 discloses battery separators comprising microporous sheets of filled polyolefins. The separators have a pore size which is generally less than 1 .mu.m in diameter. They are produced by blending polyolefin, filler and plasticizer, forming said mixture into sheet form, and subsequently extracting the plasticizer from said sheet. The preferred plasticizer is petroleum oil and the removal of the plasticizer results in the formation of pores. However, the plasticizer is usually only incompletely removed by the extracting process and the remainder may cause the formation of black deposits during battery use.
Several early proposals were made to construct a separator sheet for lead acid batteries from meltblown fibers. U.S. Pat. Nos. 3,847,676, 3,870,567, 3,972,759 and 4,165,352 disclose a process in which a polypropylene resin is meltblown into fibers or filaments to form a nonwoven mat, compacted, and then treated with a surfactant. Although the inventors of the above patents have noted the importance of high porosity, small pore size, low electrical resistance, good abrasion resistance and stiffness it was also recognized that many of these properties are antagonistic to each other. That is, that if one maximizes on of these properties, one is liable to minimize one or more of the other properties. It was suggested to use separators which preferably have a pore size within the range of 7 to 20 .mu.m and a porosity of 50 to 65%. The fibers used to produce the separators have a diameter of 0.05 to 50 microns, preferably 1 to 5 microns and 0.5 to 5 microns, respectively. These references do not indicate which proportions of fibers having a certain diameter are used. The separators typically have an electrical resistance of about 13 to 32 m.OMEGA./cm.sup.2.
U.S. Pat. No. 5,126,219 discloses battery separators comprised of a web formed of a multiplicity of microporous fibers or filaments of filled ultrahigh-molecular weight polyolefin. The web has a thickness of between about 1 and 120 mils and macropores created by the interfiber or interfilament interstices. Manufacture of the filled fibers involves the use of a plasticizer which is removed after fiber formation by extraction.
GB 1,603,635 is concerned with a method of producing battery separators which involves the preparation of a furnish containing a synthetic pulp, forming a paper web from the furnish, subjecting the paper web to heat and pressure to consolidate the paper web and to emboss the paper web. The synthetic pulp is a pulp of polyolefin fibers having fibrillated structure. The separators have a maximum pore size of 50 .mu.m and a maximum mean value of about 20 .mu.m. The above separators have proved unsatisfactory in that they do not sufficiently prevent the growth of lead dendrites through the separator between the positive and negative plates and they have consequently not been used commercially to an appreciable extend.